Hi,
Your Domain Name are registered on your web hosting or in a different website?
You have DNS settings in Webuzo but this settings is ignored if you have the domain in other hosting and you are using different name server.
Only you know where DNS of your domain are managed. If there are not managed by Webuzo you need to go in your DNS panel and configure manually the correct records for your email.
For manage a VPS if your DNS is not managed by Webuzo so your domain is outside of the VPS company you need know how to use and set DNS. You can Google and search for tutorials.
I cannot set your VPS for you.
1. I don't know well this error but you can do a Google search for know better, however I suggest to correct that only after fix other problem.
2. your DNS seems to be not correct, this is why you have issue.
3. The reverse DNS must be set by your hosting provider where you have VPS, before ask them to fix this you need learn how to use DNS , have a correct SPF for email, than when this issue are resolved you can consider to fix also RDNS